1. Field of the Invention
The invention generally relates to wall anchors and, more particularly, relates to self-drilling hollow “rock bolts” that are used to reinforce the rock walls of mine openings, tunnels, and the like. The invention additionally relates to methods of fabricating, assembling, and using such rock bolts.
2. Discussion of the Related Art
Mining and tunneling applications often require that the rocks forming the walls of the mine opening or tunnel be reinforced against both the dead weight of the rock, slow deformation and/or sudden bursting. Bolting is the most commonly-used technique for rock reinforcement in underground excavations. Millions of rock bolts are consumed worldwide every year. Basic demands of rock bolts are that they have to be able to bear not only a heavy load, but also must withstand a certain elongation before bolt failure. In highly-stressed rock masses, the rock reacts to excavation either in form of large deformation in weak rocks, or of rock bursting in hard rocks. In these situations, deformation-tolerable (or energy-absorbable) bolts are required in order to achieve good rock reinforcement and reduce the risk of rock fall. Particularly in the mining industry, this need for deformation-tolerable bolts is even stronger than in other rock branches since mining activities are getting deeper and deeper, and problems of rock deformation and rock burst are becoming increasingly severe as the depth increases.
Traditional rock bolts, however, did not provide a good combination of anchoring or load bearing ability and deformability. For example, fully grouted traditional rebar bolts offer very limited elongation (on the order of 30 mm) prior to failure. Traditional frictional bolts provide an unacceptably low load-bearing capacity for many applications, even though they exhibit high deformability.
More recently, a rock bolt has been developed that is locally anchored at one or more discrete locations and that is deformable between the anchors. This bolt, commercially available from Normet under the trade-name D-Bolt®, is disclosed in U.S. Pat. No. 8,337,120, the subject matter of which is hereby incorporated by reference in its entirety. The bolt includes a relatively smooth steel rod with a number of discrete integral anchors along its length. The bolt is anchored in a borehole with either cementitious grout or resin. The bolt is fixed within the surrounding grout primarily at the locations of the anchors, while the smooth sections between the anchors can freely deform when the bolt is subjected to rock dilation. The bolt absorbs the rock dilation energy through fully mobilizing the strength and deformation capacities of the bolt material, typically engineered steel. The smooth sections of a D-Bolt independently provide reinforcement functions to the rock, and failure of one section does not affect the reinforcement function of other sections of the bolt.
The D-Bolt rock bolt offers an excellent combination of deformability and load bearing capacity. However, it does exhibit some disadvantages in some applications.
For example, D-Bolt rock bolts and other rock bolts typically come in standard lengths, requiring that all boreholes be drilled to the same depth or, in the alternative, that different bolts of different, albeit still standard, lengths be kept on-hand to permit some versatility of reinforcement depth.
In addition, a D-Bolt typically must be grouted into a previously-drilled borehole in a three step procedure including borehole drilling, grout insertion, and rock bolt insertion. The grout typically is inserted into the borehole either by being injected directly into the borehole, or by inserting one or more grout-filled cartridges into the borehole. These cartridges are ruptured when the rock bolt is subsequently inserted into the borehole. In either event, the grout is intended to fill the space between the rock bolt and the inner peripheral surface of the borehole and, upon hardening, to lock the rock-bolt to the rock at the local anchors. However, if the rock is highly fractured, debris may form a barrier that prevents the grout from completely filling the gap between the rock bolt and the peripheral surface of the borehole. In addition, some grout takes the form of a two-part resin that must be mixed by rotation of the bolt. Debris in the borehole might hinder adequate resin mixing. In extreme situations, the borehole may effectively collapse upon removal of the drill, preventing subsequent insertion of the grout and/or the rock bolt into the borehole.
Self-drilling rock bolts are known that negate the need to drill the borehole with a separate tool before inserting the rock bolt, eliminating the risk of borehole collapse prior to rock bolt insertion and eliminating or reducing the other detrimental effects of borehole collapse around a rock bolt. The typical self-drilling bolt comes in the form of a hollow tube bearing a sacrificial drill bit at its inner end. The tube is of smaller diameter than the bit so that, upon being drilled into the substrate, a borehole is formed around the bolt. Grout then can be injected into the bolt from its outer end, whereupon the grout flows axially through the bolt, through one or more passages in or near the inner end of the bolt or the sacrificial drill bit, and outwardly between the bolt and the borehole wall to fill the gap.
However, existing self-drilling bolts, including existing self-drilling hollow rock bolts, like the other traditional rock bolts described above, lack local anchors between relatively elongateable bolt sections. Most self-drilling rock bolts instead are threaded or otherwise have relatively small anchors along their entire length and, thus, lack any sections that are more elongateable or, for that matter, offer greater anchoring ability than any other sections. Traditional self-drilling rock bolts thus do not provide an acceptable combination of local anchoring or load bearing ability and elongateability.
The need therefore exists to provide a hollow, self-drilling, locally anchored, elongateable rock-bolt.
The need still additionally exists to provide a hollow, locally anchored, self-drilling rock bolt that is of adjustable length, enhancing greater versatility of borehole depth without increasing inventory requirements.
The need additionally exists to provide a simplified process of installing a locally anchored, hollow, self-drilling rock bolt.